1. Field of the Invention
The present invention relates generally to switching power converter circuits, and more specifically, to a switching power converter in which an auxiliary winding power supply includes a selectable mode to stabilize the voltage provided from the auxiliary winding.
2. Background of the Invention
In order to supply power to control circuits of a line-powered switching power converter, a low voltage power supply is needed, typically between 3V and 12V at a few milliamperes of current. However, until the power converter is operating, the only power source typically available is the Alternating Current (AC) power line. The high voltage of the AC power line makes it impractical to use resistors to drop the voltage to the required voltage for the controller, as the power dissipation in the resistor will typically be on the order of several Watts.
Therefore, an auxiliary winding provided on one of the converter magnetics is frequently used to supply power to the converter controller integrated circuit (IC), since a lower voltage can be generated directly through the use of the auxiliary winding, therefore reducing wasted power. However, such an auxiliary power supply still has an output voltage that varies with the magnitude of the rectified AC power line at the input of the switching power converter, which can vary as much as 3:1 for a typical Universal Input power supply, and when start-up and transient hold-over conditions are taken into account, the input voltage variation is even greater. In order to ensure that there is sufficient voltage available to operate the controller IC under all input line conditions, the maximum auxiliary power supply output voltage will typically be at least three times the minimum required output voltage. Therefore, the IC must either be designed to handle the full range of power supply voltages that may be provided from the auxiliary winding or the voltage must be regulated, e.g., with a Zener diode circuit, wasting power, dissipating heat, and typically reducing reliability.
Therefore, it would be desirable to provide an auxiliary power supply circuit and method that provide operating voltage for a controller IC over a wide range of input line conditions, without an output voltage that varies over the full range of auxiliary winding output voltage, or that requires lossy regulation of the auxiliary winding output voltage.